Friday, January 21, 2005

In EN 61340-5-1:2001 the user guide suggests that parts of the standard need not be adopted if they are not relevant or needed for the processes carried out. In practice this is open to interpretation. In an electronics facility, is the use of ESD garments really necessary?

I interpret the standard as clearly saying that if you use ESD garments then they must conform to the standard. The more difficult question is, are they necessary and appropriate in your particular case? This is a very hard one to argue and in the end it often boils down to a personal judgement based on your view of your product and processes.

At one extreme if you were making a cheap "throw away" consumer product with cheap components that were not particularly ESD susceptible then I think you might not worry about using ESD garments. At the other extreme if you have a high reliability product with high cost of failure (e.g. satellite) using high susceptibility components then you would clearly take more care and use ESD garments. Most facilities operate somewhere in between.

As another consideration, if your operators all wear cotton tee shirts in a tropical climate there is likely to be little ESD risk. If they may wear long sleeved fleeces or woolly jumpers there may well be some risk from charged clothing and use of ESD garments may be advisable. Another consideration is that use of ESD garments helps to enforce ESD discipline and keep people in the right frame of mind that they should be ESD aware while wearing the coat.

Monday, January 10, 2005

I get a steady trickle of emails from all over the world from people suffering from static shocks. For some it can seemingly become a real problem. For example, getting a shock every time you touch someone can be a problem for personal relationships! I used to get so many of these emails that I wrote my on-line article "Static shocks, and how to avoid them ". This seems to help many people and attracts many visitors to our web site.

There are many possible factors and so it is not possible to say for sure what causes shocks in any particular situation without doing a first-hand analysis. However two factors are very commonly important - shoes and floor materials. Over the last 50 years or so big changes have happened in these materials - nowadays floor materials and the soles of shoes are often made from insulating materials - often plastics.

The strange thing is that not all shoes and floors give the same charge build-up problems even if they are equally highly insulating. Why should this be? The answer lies in the charge generation characteristics of the particular combination of materials. Some materials generate much more charge when rubbed together than others do. The charge build-up is the balance of charge generation rate and charge dissipation rate - if the charge is generated much more quickly than it can escape, static charge build-up occurs. If it is generated less quickly than it can escape, no build-up of static electricity is found.
A dissipative materials is one which allows the charge to escape slowly, within a few seconds. If manufacturers of shoes and flooring always made their products out of dissipative, rather than insulating materials, we would suffer far less static shocks than we do now.

Someone asked me today if there is a minimum voltage for required an electrostatic discharge (ESD) to happen. He was probably thinking of the Paschen minimum - this is a minimum voltage to break down an air gap between two electrical conductors. When two electrodes are brought closer together, the breakdown voltage between then reduces nearly linearly with distance. However a minimum occurs at about 0.1 mm - at this distance an air discharge requires about 300-350 V across the gap. If you bring the conductors closer, the breakdown voltage theoretically increases! In practice it may not, because a different type of discharge occurs at small gaps. So really there is no minimum - at the limit an ESD will occur when the conductors touch.

Sunday, January 09, 2005

Often I get calls about this time of year (January) from people who moved into a new building sometime last year. Around this time they start to really suffer static shocks to the extent they contact me to try to do something about it.

There can be many factors and many variations - but one really common one is the floor material. If the floor material is highly insulating, this is a recipe for trouble. Static charges are built up by people walking on the floor (or by cars or fork trucks driving on the floor) , especially under dry air conditions. Charge build-up quickly leads to high voltages - and ZAP!!!!

For much of the year in the UK the air is damp, and this helps to prevent excessive charge build-up. As soon as winter comes, the air can be cold outside and warm and dry inside. This is ideal for static charge build-up. The user suddenly starts to experience shocks. What can they do about it? Unfortunately many treatments are only temporarily effective. If the real problem is the floor material the real solution is to replace it - but this can be very expensive. Not the sort of thing you want to be told if you've moved into your nice new building in the last year or so....
I've visited several car parks now where the floor was covered with epoxy. A nice hard wearing coating, I'm sure, and looks good. Unfortunately a car running on epoxy charges like a Van de Graaf generator and can be at several kV voltage by the time the driver reaches to pull a ticket from the machine at the barrier..........ZAP!!! A highly charged car can pack a lot of stored energy and the shock can be very painful!

Are temperature and humidity important from a static electricity view?

Normal ambient temperatures are not usually an issue but from an electrostatic view but humidity is - electrostatic charge build-up problems increase at low humidities, especially < 30%rh. There is a link between temperature and humidity in that for a given air misture content, humidity approximately halves for a 10 degree rise in temperature. This is why ESD is often worst in winter (Jan-Mar) when the air is cold and dry outside and is brought in and warmed by about 15-20 degrees. Even high humidity cold air (e.g. 80%) can become low humidity (20%) when heated by 20 degrees if no moisture is added..

It is often under such dry air conditions that people feel static shocks, and in the electronics industry component damage may be exacerbated. I get many emails from people who are suffering shocks during dry air conditions, whether it be during winter (especially in northern climates) or in desert areas where the air is naturally warm and dry.

About Me

Dr Jeremy Smallwood has been working as a static electricity consultant since 1987. He started his company Electrostatic Solutions Ltd to give top level static electricity research and development and consultancy services in 1997.